1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM).
2. Description of Related Art
A magnetic memory, such as a magnetic random access memory (MRAM), is also a non-volatile memory, which has advantages such as non-volatility, high intensity, fast access speed, and radiation hardness. Data of logic “0” or logic “1” is recorded by the magnitude of magnetoresistance generated by the parallel or anti-parallel management of magnetic moments of magnetic substances adjacent to a tunneling barrier layer. When writing data, a common method is to use two current lines, such as a write bit line (WBL) and a write word line (WWL), to sense magnetic memory cells selected by the intersection of magnetic fields and change the value of the magnetoresistance value by changing the direction of the magnetization of the free layer. When reading memory data, current is allowed to flow into the selected magnetic memory cell, so as to determine the digital value of the memory data according to the read resistance value.
FIG. 1 is a basic structure of a magnetic memory cell. Referring to FIG. 1, in order to access a magnetic memory cell to write data, current lines 100 and 102, which are crossed and have suitable current, are required. According to the operation modes thereof, the current lines 100 and 102 may be also referred to as a bit line and a word line. The two lines carrying a current will generate a magnetic field of two directions, so as to obtain a magnetic field having a desired magnitude and direction applied on a magnetic memory cell 104. The magnetic memory cell 104 is a stack layer structure, and includes a magnetic pinned layer having a fixed magnetization or a total magnetic moment in a predetermined direction. An angular difference between the magnetizations of a magnetic free layer and the magnetic pinned layer is used to generate different magnetoresistance values to read data. Further, to write data, a writing magnetic field may also be applied to determine the direction of the magnetization of the magnetic free layer under no magnetic field. The data stored in the memory may be read by output electrodes 106, 108. The operation details of the magnetic memory is familiar to those of ordinary skill in the art and will not be described herein any more.
FIG. 2 shows a memory mechanism of a magnetic memory. Referring to FIG. 2, the magnetic pinned layer 104a has a fixed magnetic moment direction 107. A magnetic free layer 104c is located above the magnetic pinned layer 104a, and is isolated from it by a tunneling barrier layer 104b disposed between layer 104a and layer 104b. The magnetic free layer 104c has a magnetic moment direction 108a or 108b. The magnetic moment direction 107 is parallel to the magnetic moment direction 108a, so the generated magnetoresistance, for example, represents the data of “0”. On the contrary, the magnetic moment direction 107 is anti-parallel to the magnetic moment direction 108b, so the generated magnetoresistance, for example, represents the data of “1.”
The magnetic free layer 104c in FIG. 2 is a single-layer structure, and often causes wrong data in operation. In U.S. Pat. No. 6,545,906, for the free layer, a ferromagnetic/non-magnetic metal/ferromagnetic three-layer structure is used to replace the single layer ferromagnetic material, so as to reduce the interference of adjacent cells when writing data. FIG. 3 shows a structure of a magnetic memory cell, which includes a pinned stack layer 120, a tunneling layer 128, and a magnetic free stack layer 130. The pinned stack layer 120 is including a bottom pinned layer 122, a coupling layer 124, and a top pinned layer 126. The magnetic free stack layer 130 is including a bottom free layer 132, a coupling layer 134, and a top free layer 136. The material of the bottom free layer 132 and the top free layer 136 is, for example, ferromagnetic material, and the material of the coupling layer 134 is, for example, a non-magnetic metal material. The arrows in the drawing represent the directions of the magnetizations. The magnetizations of the bottom pinned layer 122 and the top pinned layer 126 form a magnetic field loop, and will not be influenced by an operation magnetic field. The magnetizations of the bottom free layer 132 and the top free layer 136 are disposed in anti-parallel, and may be influenced by an externally applied operation magnetic field, so as to change stored data. The data depends on the magnetoresistance variation caused by the magnetization between the top pinned layer 126 and the bottom free layer 132. The magnetic easy axis of the bottom free layer 132 of the memory cell and the magnetization of the top pinned layer 126 are parallel or anti-parallel to each other, shown as the pattern 150. The direction of a magnetic anisotropy axis (referred to as a magnetic easy axis for short) is represented by a double arrow, and the magnetization of the top pinned layer 126 is represented by a single arrow.
In order to reduce the interference of adjacent cells when writing data, for the free layer, the ferromagnetic/non-magnetic metal/ferromagnetic three-layer structure is used to replace the single-layer ferromagnetic structure, and the two ferromagnetic layers above and below the non-magnetic metal layer are arranged in anti-parallel. In addition, a toggle mode is used, and the WBL and WWL form an angle of 45 degrees with the magnetic easy axis of the free layer, respectively, and the provided currents are written in a certain sequence. This method may solve the problem of interference efficiently, but cause a problem that a large current is needed to write data.
When the magnetic memory is being designed towards high density, besides the magnetic field for switching must be reduced under the toggle mode, the existing problems still include that adjacent cells are more easily interfered by the magnetic field of the exposed write line when the size is miniaturized. The current free layer is a toggle MRAM structure of the SAF free layer, which is a preferable anti-interference method. However, the operation quadrants of the structures are the same in essence, in relation to the design of a magnetic tunneling junction (MTJ) element, if the exchange coupling J is too small or a bias field caused by an asymmetrical synthetic pinned layer is too large, the anti-interference capability of the MTJ element will be degraded. As a result, the magnetic memory cannot operate normally.
How to solve the interference problem to let the magnetic memory operate normally is a topic that needs further research.